The rapid growth of mobile data services caused a lot of pressure on wireless cellular network, especially the wireless access network. Now the wireless access capability needs to be enhanced urgently to meet the rapidly growing demand. At the same time, due to its inexpensive, simplicity and easy to handle, WiFi has been widely deployed worldwide. More and more client devices have the dual-mode access function of WiFi and wireless cellular at the same time, such as smart mobile phone, iPads, netbook and so on. The increased availability of the dual-mode terminal is to integrate WiFi and 3GPP wireless access (3G/4G) into a multimode wireless access environment, and the rapid growth of the quantities of dual-mode terminals makes it possible that improving wireless access capacity by converging the two access technologies of WiFi and the wireless cellular access technology (3G/4G).
A solution of establishing the multimode wireless access network is to integrate a WiFi access point and a wireless cellular network base station in the same equipment, and access a mobile core network through a common backhaul line. Typically, this multimode wireless access system is a kind of overlapping heterogeneous network. A cellular network provides complete coverage to provide mobility support, while in the locations where WiFi signals are available, for example in a hot spot or hot zone, WiFi is used to enhance the wireless access capability.
In this converged multimode wireless access network, it is an important problem to be solved that how to provide safe access control and secure data transmission in the WiFi wireless link.
The typical WiFi access procedure is illustrated in FIG. 1, which can be divided into three phases: scan, authentication and association.
In order to discover a WiFi AP (Access Point), firstly a WiFi station (STA) performs a link layer procedure called “scan”. In 802.11, there are two scan methods: passive and active. In passive scanning, the STA listens for beacon frames transmitted by the APs at regular intervals. The beacon comprises the information such as SSID, supported rates and security parameters and so on. The STA can also obtain the same information by using active scanning. During the active scanning (or probing), the STA transmits probe request frames, and after receiving the probe request frames, APs will respond with the probe response frames comprising the information similar to that comprised in the beacon frames in the passive scanning. Thus the STA can collect the information about the candidate APs and selects one of them. The AP's selection depends on some factors, such as signal quality, access network capabilities, user preferences and so on. After selecting an AP, the STA will proceed the access authentication phase. Once the authentication is successfully completed, the STA will finally negotiate with the new AP the communication data rate and the reserved resources through the association procedure. The AP finally replies with an association response including the supported data rates and the session ID.
At present, the WiFi access authentication comprises two major mechanisms: the preshared key authentication and the access authentication based on 802.1x. The preshared key authentication requires presetting the key between the two communication parties before the key is used, and the AP and all the related clients share the same key. The authentication process is illustrated in FIG. 1. Firstly, the WiFi STA sends the authentication request to the AP. Once the AP receives the request, it will reply with an authentication frame comprised of a 128 octets of challenge text that is generated in the random way. Next, the STA will copy the challenge text into an authentication frame and encrypt it with the shared key, then send the encrypted frame to the AP. In the fourth step, the AP will decrypt the returned challenge text using the same shared key and compare it to the original copy sent earlier. If a match occurs, then the AP will reply with an authentication response indicating an authentication success; if not, the AP will send an authentication failure indication. The shared key authentication solution is very simple, and it does not require the involvement of other equipments. However, the key needs to be shared among the AP and all the clients in advance, which makes the solution more fit the home or small offices with limited number of users. For a large scale network with thousands or tens of thousands of APs and wireless stations, it will result in a huge amount of work to share the keys. And it is not easy to dynamically change the key. Furthermore, the solution is vulnerable since the keys are the same in all related STAs and the APs. Therefore, this method is not applicable to the large scale network deployment in an operator level.
The second typical authentication method is based on 802.1x. The process is illustrated in FIG. 2. The communication standard between the AP and AS (Authentication Server) is RADIUS (Remote Authentication Dial In User Service). The authentication session between the STA and AS is carried in EAP (Extensible Authentication Protocol) frames. The EAP frames are carried as EAPOL (EAP over LAN) in 802.1x, and are carried as EAP message parameters in RADIUS.
In the solution based on shared key, after the association, the STA will be able to gain network access. In the solution based on 802.1X, however, this is not enough. In order to transmit data frames through an established association, the STA must unlock the 802.1X port mapped to the newly created association. In order to do that, the STA will perform EAP authentication using 802.1X frames.
The EAP authentication is initiated when the STA transmits an EAPOL-Start frame, or when the AP transmits an EAP-Identity request frame. The STA then uses the authentication credentials and a certain EAP authentication method to authenticate with a back-end EAP server. The AP acts as a pass-through entity by forwarding EAP packets from the STA to the back-end EAP server and vice versa. EAP packets are extracted from 802.1x frames and encapsulated in RADIUS messages, then sent to the back-end EAP server, which extracts the EAP payload from the message and sends the reply message to the STA through the AP.
After multiple signaling interaction between the EAP and the STA (the detailed procedure depends on the EAP authentication method in use), the EAP server determines whether it has successfully authenticated the STA identity. In case of a success, the EAP server transmits an EAP-Success message. The key information may be sent to the STA and the AP as well. The AP then unlocks the 802.1x port mapped to the associated STA. The unlocked 802.1x port allows only protected and authenticated data frames to pass through.
The authentication method based on the 802.1x can provide the centralized authentication and key management for the WiFi clients. However, it is a complex method that requires the introduction of the EAP/Radius server as well as supporting the 802.1x protocol in both the WiFi clients and the AP. In addition, for operators, in order to facilitate the maintenance and to lower down the cost, the WiFi authentication must be integrated with the cellular network. Therefore, the cellular network elements, e.g. HLR/HSS need to support the inter-operation with the WiFi authentication server, which increases the system complexity further.
In a summary, the pre-shared key authentication method has low safety, is vulnerable, difficult to maintain and does not fit the large scale deployment. While the authentication method based on 802.1x needs to introduce the EAP/Radius server as well as the support from the WiFi clients and the AP, whose cost and complexity are high. In addition, the inter-operation with the cellular network further increases the complexity of the solution.